Use of peroxidase in immunohistochemistry and in immunoassay will be pursued. Fixatives will be designed to obtain the maximum in preservation of antigenicity and tissue ultrastructure. Peroxidase will be used as the label in competitive immunoassay in place of radioactive tracers. Enzyme as a label should avoid the need of specialized counting equipment and the hazard and care of radioactive tracers. Studies on the mode of cellular differentiation in the anterior pituitary gland in vivo and in vitro will be continued. Conditions and factors necessary for functional cellular differentiation of the glands of embryos, and of neoplastic tissue of anterior pituitary gland will be studied using the peroxidase labeled antibody method and peroxidase-immunoassay. Antibodies against tissue specific chromatin acidic-proteins will be used to study the role of the acidic protein in nuclear differentiation and functional cytoplasmic differentiation.